Generally, a principle of solvent extraction is known as follows.
When one solvent containing a solute is in contact with another solvent that does not contain the solute (or contains only a small amount of the solute), the solute moves from one solvent to another solvent up to the time when solute concentrations in the solvents are balanced with each other.
As described above, liquid-liquid extraction denotes a unit operation whereby a solute is transferred from one solvent to another solvent to be separated by the solubility difference between the two solvents that are immiscible.
A conventional operation of the liquid-liquid extraction is a countercurrent operation of two liquids, namely an operation of the two liquids being moved in opposite directions to be in contact with each other.
In an extraction of a conventional extraction column extending vertically, the two liquids are a light phase liquid and a heavy phase liquid.
In the meantime, after completing the extraction, in order to separate desired solute and solvent, first, it is desired to separate the solvent containing the solute and the solvent that lost the solute. Here, generally, a specific gravity difference between the two solvents is used in the conventional extraction column.
The conventional extraction column may be understood by referring to FIG. 1, which shows a schematic view of a liquid-liquid extraction column.
The conventional liquid-liquid extraction column in FIG. 1 includes an upper tank 20 and a lower tank 30 respectively provided at an upper end and a lower end of the column body 10. After extracting the solute from fluid A and fluid B, fluid A and fluid B are separated, and are respectively discharged from the lower tank 30 and the upper tank 20.
The upper tank 20 and the lower tank 30 have respective inlet pipes 21 and 31 and have respective outlet pipes 22 and 32 through which the fluids are introduced and separated fluids are discharged.
Here, a control valve 32a is provided at the outlet pipe 32 of the lower tank 30. The control valve controls outflow rate to maintain the level of an interface between the two separated fluids having different specific gravities in the upper tank 20.
With such configuration, through a series of processes in the column body 10, the fluid A having high specific gravity is discharged from the lower tank 30 through the outlet pipe 32. The fluid B having low specific gravity is discharged from the upper tank 20 through the outlet pipe 22.
Here, the fluid B having low specific gravity accumulates due to the level of the interface, and thus, fluid B is naturally discharged through the outlet pipe 22. Fluid A having high specific gravity is artificially discharged by the controlling of the control valve 32a. 
In the meantime, in a continuous process using the conventional liquid-liquid extraction column, in order to separate the two fluids after the extraction, it is desired to maintain the point at which the two fluids are separated during the process, namely, the level of the interface between the two fluids.
In the process of separating the two fluids having different specific gravities after extracting the solute, when the level of the interface between the fluids A and B is excessively increased, the fluid A may be discharged from the upper tank 20 through the outlet pipe 22. Therefore, it is desirable to maintain the level of the interface between the fluids A and B having different specific gravities in the upper tank 20 within a certain range.
To this end, a measuring device 40 measures the level of the interface in the upper tank 20. The control valve 32a of the lower tank 30 controls the outflow rate of the fluid A. Therefore, the level of the interface in the upper tank 20 is maintained or controlled by adjusting the total level of the fluids in the extraction column.
Here, as a method of measuring the level of the interface in the upper tank 20, there is a method of using a density measuring device or a level switch, etc. to check whether or not the level of the interface is higher than a preset level, or a method of using a level transmitter for measuring the interface, or a method of using pressure difference between the upper-lower ends of the upper tank 20 to continuously measure the level of the interface, etc.
Here, theoretically, inflow rate is steady in an equilibrium state. Therefore, when the outflow rate is steadily maintained, the level of the interface is required to be steadily maintained. However, in a real operation, it is difficult to precisely and steadily maintain the inflow rate. Moreover, slight changes of the level of the interface, or errors caused by electromagnetic interferences in a control system, etc. are used for feedback. Therefore, an opening ratio of the control valve 32a continuously changes such that it is difficult to steadily maintain the outflow rate.
Consequently, it is difficult to steadily maintain the level of the interface in the upper tank 20.
Moreover, the life of the control valve 32a is shortened by the continuous operation of the control valve 32a. 
In addition, various measuring devices 40 are used for the interface measurement as described above. In order to transmit and receive measurement signals between the control valve 32a and the measuring devices 40, a complex control system is required.
Furthermore, since the control system is complex as described above, the demand for maintenance increases, thereby increasing maintenance costs. In addition, work efficiency decreases due to increased man hours.